Standard for wireless local area network (WLAN) technology has been developed as the institute of electrical and electronics engineers (IEEE) 802.11 standard. IEEE 802.11a and 802.11b use an unlicensed band at 2.4 GHz or 5 GHz. IEEE 802.11b provides a transmission rate of 11 Mbps and IEEE 802.11a provides a transmission rate of 54 Mbps. IEEE 802.11g applies orthogonal frequency-division multiplexing (OFDM) at 2.4 GHz to provide a transmission rate of 54 Mbps. IEEE802.11n applies multiple input multiple output (MIMO)-OFDM to provide a transmission rate of 300 Mbps for four spatial streams. IEEE 802.11n supports a channel bandwidth of up to 40 MHz to provide a transmission rate of 600 Mbps.
IEEE 802.11af, which specifies an operation of an unlicensed device in a TV white space (TVWS) band, is being developed.
TVWS includes an ultra high frequency (UHF) band and a very high frequency (VHF) band as a frequency allocated to a broadcast TV and refers to a frequency band in which use of an unlicensed device is permitted under the condition that it does not hinder communication of a licensed device operating at a corresponding frequency band. The licensed device includes a TV, a wireless microphone, etc. The licensed device may be called an incumbent user or a primary user. In addition, in order to overcome a problem in terms of coexistence between unlicensed devices using a TVWS, a signaling protocol such as a common beacon frame, a frequency sensing mechanism, etc. may be required.
Operations of all unlicensed devices are permitted in frequency bands of 512 to 608 MHz and 614 to 698 MHz except in a few special cases. However, in frequency bands of 54 to 60 MHz, 76 to 88 MHz, 174 to 216 MHz, and 470 to 512 MHz, only communication between fixed devices is permitted. Fixed devices refer to devices which transmit signals only in a determined location. An IEEE 802.11 TVWS terminal refers to an unlicensed device operating using an IEEE 802.11 media access control (MAC) layer (or a second layer L2) and a physical layer (PHY) (or a first layer L1) in a TVWS spectrum.
An unlicensed device which desires to use a TVWS should provide a protection function for a licensed device. Accordingly, the unlicensed device must confirm whether the licensed device occupies a corresponding band before starting signal transmission in the TVWS.
To this end, the unlicensed device may confirm whether a corresponding band is being used by the licensed device by performing spectrum sensing. A spectrum sensing mechanism includes energy detection and feature detection. When strength of a signal received through a specific channel is above a prescribed value or when a DTV preamble is detected, the unlicensed may determine that the licensed device is using the specific channel. When it is determined that the licensed device is using a channel immediately adjacent to a currently used channel, the unlicensed device should lower transmission power thereof.
In addition, an unlicensed device may obtain available channel list information in a corresponding area by accessing a geolocation database (DB) through the Internet or a dedicated network. The geolocation DB stores and manages information about registered licensed devices and information about available channels which dynamically vary according to geographic location and channel use time of the licensed devices.
In this specification, a whitespace band includes, but is not limited to, the aforementioned TVWS. In this specification, the term ‘whitespace band’ refers to a band in which an operation of an unlicensed device is permitted only when an operation of a licensed device is preferentially permitted and the licensed device is protected. In addition, a white space device refers to a device that operates in a whitespace band. A device according to an IEEE 802.11 system may also be an example of the whitespace device. In this case, the whitespace device may refer to an unlicensed device that operates using IEEE 802.11 medium access control (MAC) and a physical layer in a whitespace band. That is, typical AP and/or STA according to 802.11 standard, which operates in a whitespace band, is an example of an unlicensed device.